1. Field Of The Invention
This invention relates to a magnetic shield utilizing a superconducting phenomenon and, more particularly, to a magnetic shield in which a superconductor is composed of bismuth high temperature oxide superconductor for accurately measuring a magnetoencephalographic wave or other external alternating magnetic field to be measured.
2. Description Of The Prior Art
Magnetic shields generally have an active shield and a passive shield. The passive shield has a ferromagnetic shield/and a superconducting shield known per se. The characteristics of the respective magnetic shields have features. (See "Cryogenic Engineering" written by Ogasahara, pages 135-147, Vol. 18, No. 4, 1978.)
Among the magnetic shields, the ferromagnetic shield eliminates introduction of a magnetic field into an inner space by absorbing an external magnetic field in a ferromagnetic material such as, for example, a nickel alloy commercially available as Permalloy, trade name. In this case, the external magnetic field which is not absorbed by the ferromagnetic material remains in the inner space. That is, the external magnetic field cannot be fully eliminated by the ferromagnetic shield. On the other hand, the superconducting shield excludes an external magnetic field by reflecting it toward the exterior without absorbing it in a superconductor to obviate filly the introduction of the strong magnetic field into the inner space, and its magnetic shielding effect is much larger than that of the ferromagnetic material. More specifically, the superconducting shield can completely exclude the external magnetic field, while; the ferromagnetic shield has a limit due to the presence of the magnetic field remaining in the inner space. A magnetic shield of a range exceeding the limit is expected by a superconducting magnetic shield. However, since the critical temperature Tc at which an electric resistance becomes zero, of an Nb metal superconductor of, for example, Nb.sub.3 Sn, Nb.sub. 3 Ge, etc., is approximately 20 K of cryogenic temperature, it is necessary to employ liquid helium as a refrigerant. Thus, construction of a magnetic shield has a barrier of its cost, and the superconducting shield is not yet realized except an extremely small field at present.
Recently, researches for trying an analysis of a mechanism of human brains, a headache, a diagnosis of brains, etc., have been activated by measurements in magnetic waves generated from the brains. Heretofore, searches in the interiors of human brains by MRI, positron CT, etc. have been clinically executed, but limited in its resolution, radioactive rays to be used, etc. Therefore, needs for detection of magnetoencephalographic waves themselves are abruptly raised.
However , the intensity of the magnetoencephalographic waves is 10.sup.-3 gauss, extremely weak, while the magnetic field of the earth or the terrestrial magnetism is as strong as 0.3 gauss. There are large magnetic noises generated from various electric devices and equipments in addition to the magnetic fields of the earth in the environment.
It is required for a magnetic sensor of high sensitivity and a magnetic shield for shielding the magnetic field of the earth and the magnetic noises under such circumstances to detect extremely weak magnetoencephalographic waves. To this end, a magnetic sensor called SQUID (superconducting quantum interference device) or a superconducting magnetic shield has been recently developed. However, since the superconducting magnetic shield necessitates expensive liquid helium as its refrigerant as described above, it is not yet realized.
The development of a practical superconducting magnetic shield is desired not only to detect an extremely weak signal of a biomagnetism but to protect Josephson elements, IC circuits against external magnetic noises as described above.
It is an object of the present invention to provide a superconducting magnetic shield or a magnetic shielding apparatus which can shield various external magnetic noises by using inexpensive liquid nitrogen without expensive refrigerant such as liquid helium, etc.